Here is some reading material on GPS tracking.

http://www.trackwhatmatters.com/blog/labels/vehicle%20tracking.html

http://www.trackwhatmatters.com/blog/blogger.html

http:// www.fleetboss.com/needssurvey.asp

http:// www.gpsfleettracking.tv/faq.htm (this one claims a three month return on investment)

http:// www.securitymagazine.com/CDA/Articles/SEC_Highlights/BNP_GUID_9-5-2006_A_10000000000000207369